1. Field of the Invention
This invention relates to sintered bodies of alumina and like hard substances, and particularly to a sintered, hard material having improved ductility by virtue of a content of zirconia or hafnia.
2. Description of the Prior Art
It is known from the published German application 2 549 652 that the ductility of a sintered body of alumina can be improved by creating microfissures which permit some plastic deformation of the body prior to ultimate failure under applied stresses, particularly bending stresses. The microscopic fissures are generated by dispersing in the alumina, prior to sintering, agglomerates of zirconia, and sintering above the temperature at which the zirconia is converted to the tetragonal crystal form which is unstable or metastable at ordinary room temperature in the absence of stabilizing agents. The formation of microfissures in useful numbers requires the zirconia to be present in the sintering mixture and in the sintered product in the form of agglomerates having an average size of 2 to 15 .mu.m, and in an amount of 8 to 25 percent by volume. An increase in zirconia content produces an increase in ductility up to an optimum value which depends on the size of the individual particles in the agglomerates. Regardless of the particle size, the flexural strength decreases with the amount of the agglomerates, that is, with the number and size of microfissures in the sintered matrix.